Magnesium base alloys



Patented May 8', 1934 UNITED STATES MAGNESIUM BASE ALLOYS Manley E. Brooks, Midland, Mich., The Dow Chemical Company, Midland,

assignor to Mich.,

a corporation of Michigan No Drawing. Application April 21, 1932, Serial No. 606,761

8 Claims.

together with their high tensile strength and,

other useful properties, make these metals valuable materials for use particularly in aeroplane and automotive vehicle construction, for example, where reduction in weight is a prime consideration. In many such applications the resistance to corrosion of these alloys becomes a factor of much importance. In view of the importance of the corrosion resistance in these structural metals, many attempts have been made to produce magnesium base alloys having a high corrosion resistance which simultaneously possess good mechanical properties.

Heretofore it has been recognized that the coexistence of high tensile strength and a low corrosion rate in a given magnesium alloy depends upon the proportions, as well as the kind, of alloying ingredients employed. In general, it is known that alloying moderate amounts, for example, up to about 10 per cent, of the metals zinc, cadmium, tin, or up to about 2 per cent of manganese or silicon, with magnesium, .not only imparts improved mechanical properties to the resulting alloy, but also improves the corrosion resistance, while similar amounts of other elements, such as aluminum, copper, nickel, decrease the corrosion resistance although the mechanical properties are improved. The actual values of the tensile strength and corrosion rate may be varied over a considerable range by varying the proportions of any given combination of the metals selected as alloying ingredients. The compositions of the commercial mag nesium alloys containing in excess of 75 per cent of, magnesium, therefore, in many cases represent a compromise between high values for mechanical properties such as tensile strength and a high corrosion resistance.

These considerations raise the problem of producing corrosion resistant magnesium alloys in which the mechanical properties are not directly dependent upon the ingredients or proportions of alloying elements employed to improve the corrosion resistance. Inasmuch as many magnesium alloys have been developed and are well known to those skilled in the art, my invention has -for its object the improvement of the corrosion resistance of any such 6 magnesium alloy without materlaily changing the value of its mechanical properties, appearance, or machinability.

To the accomplishment of the foregoing and related ends, the invention, then, consists of the methods and alloys hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail several types of alloys and methods illustrating, however, but several of the various ways in which the principle of the invention may be used.

My invention is based upon the discovery that arsenic improves the corrosion resistance of magnesium alloys. Furthermore, I have found .75 that improvement in corrosion resistance may be obtained withouta detrimental alteration of the mechanical properties of the alloys containing in excess of per cent of magnesium. Accordingly, my invention embodies the method of improving the corrosion resistance of magnesium alloys which consists in adding arsenic thereto, and the new and improved alloy compositions thereby obtained. It is convenient to 7 use the commercial magnesium alloys as a starting point to make my new alloys, although any magnesium alloy having a suitable composition to meet the particular application in view, may be treated with arsenic according to my invention.

The quantity of arsenic which may be added to such alloys may vary from 0.01 to 5 per cent or more. I normally prefer, however,'to employ smallquantities of the same, that is, approximately from 0.1 to 1 per cent of arsenic, in most instances.

Examples of magnesium alloys to which arsenic has been added are. given in the table which shows the improved resistance {to corrosion obtainable. The corrosion resistance was determined by testing the alloys in a 3 per cent sodium chloride solution employing the well known alternate immersion method. For comparison, similar alloys without the addition of arsenic were tested at the same time and in a similar manner. The results as hereinafter reported are given in terms of the ratio between the corrosion rate (calculated from the loss in weight per unit area per day) of magnesium alloys without arsenic and the rate for similar alloys with purposes, in the table the relative corrosion rate of the alloys containing arsenic is set at unity and the corrosion rate of corresponding alloys without arsenic is given as the ratio abovementioned.

Table Comparative corrlostioniiatk re a we use Alloy composition in weight per unit area per day 1. Al 4%, Mn 0.3%, balance Mg 2 1A. Ditto (l)+As 02-05% 1 2. Al 6%, Mn 0 3%, balance Mg 2 2A. Ditto (2)+AS 0.1-0.8% 1 3. Al 8%, Mn 0.203%, balance M 2 3A. Ditto )+As 0.l0.8% l 4. A1 2-8%, Zn 0.5-6%, Mn 0.3%, balance Mg 2 4A. Ditto (4)+As 0.02% l 5. Cd l-10 balance Mg 2.1 5A. Ditto (5 +AS 0.9% 1 6. Sn l10%, balance Mg 3. 5 6A. Ditto (6)+As 0.5%.-. l 7. Zn l10%, balance Mg 4 7A. Ditto (7)+As 0.2% l 8. Cu l10%, balance Mg 2 8A Ditto (BH-As 0.4 1 9. About 1.5% Mn, balance M 1.1 9A. Ditto (9)+As 0.6% l 10. Al l-10 Mn 0.50.2%, balance Mg 2 10A. Ditto 10)+As 0.1-0.8% ,1

The beneficial effect of arsenic on the resistance to corrosion of magnesium alloys varies with the composition of the alloy to which arsenic is added. For example, according to tabulated corrosion tests, the addition of 0.6 per cent of arsenic to the binary alloy of magnesium with 1.5 per cent of manganese increases the corrosion resistance of the alloy approximately 10 per cent, while 0.2 per cent of arsenic added to a binary alloy of magnesium with from 1 to 10 per cent ofi/zinc increases the corrosion resistance about 400 per cent. In the case of other magnesium alloys containing from 1 to 10 per cent of aluminum, cadmium, or zinc, the corrosion resistance is'increased about 100 per cent or more by adding relatively small amounts of arsenic. Similar improvements in corrosion resistance may be obtained with other magnesium alloys, such as those containing up to 10 per cent of copper or tin or up to 2 per cent of silicon.

Magnesium alloys whose composition adapts them to the known processes of heat treatment or mechanical working, such as forging, rolling, and extrusion, may be submitted to these processes when arsenic has been added thereto in accordance with my invention. In some instances my invention has a-beneficial effect upon the mechanical properties. For example, the addition of from 0.4 to'0.7 per cent of arsenic to pure magnesium, in addition to increasing the corrosion resistance, also increases the tensile strength and yield point in the extruded condition 10 and 30 per cent,respectively.

The addition of arsenic to magnesium alloys is effectively accomplished by melting the alloy under a suitable flux, such as those disclosed in United States Patent No. 1,476,192, and adding arsenic by stirring the same into the molten alloy. Alloying arsenic with magnesium or magnesium alloys may be effected also by melting the metals with arsenic in a closed melting chamber without a flux or in an inert atmosphere. Arsenic-rich magnesium alloys may be made in this manner which may be employed as hardeners to make arsenic-containing alloys. Arsenic may be alloyed with magnesium a d itS the addition of this element. For comparative alloys conveniently also by reacting arsenic trioxide or pentoxide with the molten metal under a flux, since magnesium will reduce the oxides of arsenic to elementary arsenic and the flux purifies the alloy so formed from oxide of magnesium.

While I have described my invention more particularly in terms of the examples given of structural magnesium alloys, that is, alloys containing in excess of per cent of magnesium and at least one of the metals aluminum, cadmium, copper, manganese, silicon, tin, zinc, it is to be understood that the invention is not limited thereto inasmuch as other compositions of such alloys than those disclosed, such as the various binary, ternary, quarternary, and more complex types known to those skilled in the art, may be improved effectively by the addition of from 0.01 to 5 per cent, or preferably 0.1 to 1 per cent, of arsenic according to my invention. Furthermore, my invention is applicable to magnesium alloys having a lower magnesium content thanv the structural alloys. For example, the brittle alloys containing in excess of 50 per cent of magnesium may have their corrosion resistance improved by adding thereto from 0.01 to 5 per cent of arsenic. Preferably, however, from 0.1 to 1 per cent of arsenic may be employed also with such alloys.

Other modes of applying the principle of-my invention may be employed instead of the one explained, change being made as regards the proportions of the ingredients employed within the limits specified, provided the ingredients stated by any of the following claims or the equivalent of such stated ingredients be employed.

I' therefore particularly point out and distinctly claim as my invention:

1. The method of improving the corrosion resistance of magnesium alloys containing in excess of 50 per cent of magnesium, the balance consisting chiefly of a metal from the group consisting of aluminum, cadmium, copper, manganese, silicon, tin and zinc, which consists in adding to such alloy from 0.01 to 5 per cent of arsenic.

2. The method of improving the corrosion resistance of magnesium alloys containing in excess of 50 per cent of magnesium, the balance consisting chiefly of a metal from the group consisting of aluminum, cadmium, copper, manganese, silicon, tin, and zinc which consists in adding to such alloy from 0.1 to 1 per cent of arsenic.

3. The method of improving the corrosion resistance of magnesium alloys containing in excess of 75 per cent of magnesium, the balance consisting chiefly of a metal from the group consisting of aluminum, cadmium, copper, manganese, silicon, tin, and zinc, which consists in adding to such alloy from 0.01 to 5 per cent of arsenic.

4. The method of improving the corrosion resistance of magnesium alloys containing in excess of '75 per cent of magnesium, the balance consisting chiefly of a metal from the group consisting of aluminum, cadmium, copper, manganese silicon, tin, and zinc, which consists in adding to such from 0.1 to 1 per cent of arsenic.

5. The method of improving magnesium and magnesium alloys containing in excess of 50 per cent thereof which consists in adding from 0.1 to l per cent arsenic to such alloy.

6. The method of improving magnesium and magnesium alloys containing in excess 01?.50 per 8. A magnesium base alloy containing from 0.1 to 1 per cent of arsenic, from 0.3 to 10 per cent of at least one metal selected from the group consisting of aluminum, cadmium, copper, man ganese, silicon, tin, and zinc, the balance being magnesium in amount in excess of '75 per cent of the total, such alloy being characterized by having a greater resistance to corrosion than an alloy of similar composition which contains no arsenic.

MANLEY E. BROOKS. 

